1. Field of the Invention
The present invention relates to distance measuring equipment (DME) that measures the distance between an aircraft and a ground apparatus. More particularly, the invention relates to a DME ground apparatus that retransmits a pulse signal in response to a signal coming from an interrogator mounted on the aircraft, upon lapse of a preset delay time.
2. Description of the Related Art
The distance measuring equipment (hereinafter referred to as DME apparatus is a secondary radar system composed of an airborne apparatus and a ground apparatus. The airborne apparatus is mounted in an aircraft. The ground apparatus is provided on the ground and communicates with the airborne apparatus.
The DME apparatus has a transmitting-receiving device called an interrogator as a DME airborne apparatus (hereinafter referred to as airborne apparatus) on the aircraft and a transmitting-receiving device called a transponder as a DME ground apparatus on the ground side.
The interrogator provided in the airborne apparatus transmits interrogation pulses (pair pulses) of UHF band toward the transponder provided in the DME ground apparatus. (The interrogator and the transponder keep communicating with each other, with a frequency difference of 63 MHz, because the frequency allocated to the interrogator is 1,025 to 1,150 MHz and the frequency allocated to the transponder is 962 to 1,213 MHz.) The distance between the aircraft and the ground apparatus is measured from the time that elapses until the interrogator receives response pulses (pair pulses) from the transponder after it has transmitted the interrogation pulses (pair pulses). (See, for example, Japanese Patent No. 2,629,612).
A specific frequency is allocated to each DME ground apparatus. The DME ground apparatus can therefore be identified with the frequency allocated to it.
The DME ground apparatus can respond to the interrogations from a plurality of interrogators (aircrafts). The DME ground apparatus can give about 100 aircrafts distance data. Nonetheless, the ground apparatus neglects weak electric waves coming from far-off aircrafts if about 100 or more aircrafts make an access to it. Thus, the DME ground apparatus would not be over-loaded.
The channels assigned to the interrogators provided in the DME ground apparatuses, respectively, differ by 1 MHz one from another. Therefore, the DME ground apparatus is required absolutely not to malfunction even if it receives signals of adjacent channels. If the DME ground apparatus malfunctions, it will give wrong position data to the aircrafts. Should it happen, safety could not be ensured at all.
In view of this, the DME ground apparatus has, as required by law, the standardized function of not processing any interrogation signal it has received, if the frequency of the signal deviates, by 900 kHz or more, from the frequency of any allocated channel.
Hitherto, the function of not processing any interrogation signal whose frequency deviates, by 900 kHz or more, from the frequency of any allocated channel has been implemented by an analog circuit. The circuit configuration of the DME ground apparatus inevitably has a large scale. Much time must be spent in adjusting the DME ground apparatus.
Some technical publications are available, which disclose the gain control performed on DME ground apparatuses. (See, for example, Japanese Patent No. 2,629,612).
As pointed out above, the channels assigned to the interrogators provided in the DME ground apparatuses, respectively, differ by 1 MHz one from another. Therefore, the DME ground apparatus is required not to malfunction even if it receives signals of adjacent channels, in order to ensure safety.
Hence, the DME ground apparatus has the function of not processing any interrogation signal whose frequency deviates, by 900 kHz or more, from the frequency of any allocated channel.
As described above, the function of not processing any interrogation signal whose frequency deviates, by 900 kHz or more, from the frequency of any allocated channel has hither to been implemented by an analog circuit. The circuit configuration of the DME ground apparatus is inevitably large, and much time must be spent in adjusting the DME ground apparatus.
Japanese Patent No. 2,629,612 indeed describes the gain control performed in a DME ground apparatus, but is silent about any method or device of not processing any interrogation signal whose frequency deviates from any allocated channel by 900 kHz or more.